1. Field of the Invention
The present invention relates generally to a piston type compressor that comprises a drive shaft, a cylinder block having a plurality of cylinder bores arranged around the drive shaft, and a plurality of pistons, which are retained in the cylinder bores and reciprocate with the rotation of the drive shaft. More particularly, this invention relates to a gas venting mechanism in a piston type compressor which is suitable for an air conditioner in a vehicle.
2. Description of the Related Art
A piston type compressor disclosed in, for example, Japanese Unexamined Patent Publication No. 3-92587, is provided with a cylinder block having a plurality of cylinder bores formed therein and pistons which both reciprocate in the cylinder bores and which define PG,3 compression chambers in the bores. Each compression chamber is connected to a suction chamber formed in the compressor by means of a suction port. These suction ports are opened and closed by flapper type valves disposed in the compression chambers. Refrigerant gas in the suction chamber is drawn into the chamber through the corresponding flapper type valve which is forced open during the suction stroke of the piston moving from the top dead center to the bottom dead center. During the discharge stroke, when the pistons move from the bottom dead center to the top, dead center, the suction ports are closed by the flapper type valves. Refrigerant gas, compressed in the compression chamber, causes a discharge valve in the discharge port to open, which allows the gas to be exhausted through a discharge port into the associated discharge chamber.
The flapper type suction valves are opened and closed by the pressure difference between the compression chambers and the suction chamber. When the pressure in the suction chamber is higher than in the compression chambers, as occurs during the suction stroke of the pistons moving from the top dead center to the bottom dead center, the flapper type suction valves are bent or deformed to open the suction ports. The flapper type valves will not open unless the pressure in the suction chamber becomes higher by some degree than that in the compression chambers. The force needed to elastically deform the flapper type valves effectively diminishes the total suction force produced during the suction stroke (this diminishing force, will for convenience, be hereinafter referred to as suction resistance). This introduces a timing delay respecting the opening of the flapper type valves.
In conventional compressors a lubricating oil mist is normally suspended in the refrigerant gas and supplies the internal parts of the compressor with lubrication. The lubricating oil can be carried to wherever the refrigerant gas flows, and predictably sticks between the flapper type valves and the surface of a valve plate to which the flapper type valves come in close contact. The adhesive property of the lubricating oil between the surface of the valve plate and flapper type valve causes a further delay in opening action of the flapper type valves. This delay in the opening of the flapper type valves reduces the flow rate of the refrigerant gas into the compression chambers, that is, it reduces the volumetric efficiency of the compressor. In addition, even when the flapper type valves are opened, the elastic resistance of the flapper type valves contributes to the overall suction resistance of the gas flow into the compression chambers.
Conventional compressors of this type also tend to exhibit gas leakage from the compression chamber to the suction chamber. Normally during a compression stroke, refrigerant gas leaks from the compression chamber to the suction chamber through a slight clearance between the outer surface of the piston and the inner wall of the associated cylinder bore. This gas leakage reduces the amount of gas discharge from the compression chamber to the discharge chamber, thus reducing the volumetric efficiency.